1. Field of the Invention
The present invention generally relates to mitigating the effects of external magnetic fields on cathode ray tube (CRT) displays, and more specifically to a closed loop system which adjusts the drive current through a compensation coil to buck the external magnetic field.
2. Description of the Related Art
Existing CRT displays suffer intolerable display degradation in the presence of even moderate external magnetic fields e.g., greater than approximately 0.5 Gauss. The earth's magnetic field provides an ambient field which can be elevated by the presence of ferrous metals. Very high fields, of up to 5 Gauss with gradients of approximately 1 Gauss per foot, exist aboard naval ships and in industrial environments.
External magnetic shields, i.e., high permeability housings, surrounding a CRT typically provide adequate shielding against external magnetic fields up to 5 Gauss that are oriented orthogonal to the CRT viewing axis. The high permeability shield shunts most of the incident field, which prevents the field from deflecting the CRT's electron beam. For monochrome displays, external magnetic fields aligned parallel to the CRT viewing axis rotate the image about its center point, causing misregistration of the display and loss of display at the corners. Color CRT displays are even more susceptible to external fields. In addition to rotation, color displays lose color purity, beam convergence and suffer increased luminance variation.
FIG. 1 illustrates a section of a known color CRT 10 that includes an electron gun 11 for generating a beam 12 that produces a red image on a phosphor coated face plate 13. An external magnetic shield 15 is positioned around the CRT. In the baseline earth field or low level magnetic fields, the red beam is aligned to land on and excite red phosphor dots 16 to produce red images. The CRT also includes green and blue electron guns for producing beams which land on and excite green and blue phosphor dots 17 and 18, respectively.
An elevated external magnetic field 19 is shown oriented parallel to the display's viewing axis 20. In this example, the magnetic field deflects the beam so that it lands on and excites the green phosphor instead of the red phosphor. In general, the beam deflection increases as the strength of the magnetic field increases. Similar errors are induced in the green and red images such that the color and intensity of the displayed image is distorted.
It is known that the harmful effects of a slowly varying (less than approximately 2 hz) external magnetic field can be reduced by providing a bucking field in front of the CRT. Attempts to measure the external field and regulate the bucking field have been of limited success. The bucking field that opposes the external field may actually distort the CRT display.
U.S Pat. No. 4,996,461, "Closed Loop Bucking Field System" to Bentley and assigned to Hughes Aircraft Company, the assignee of the present invention, discloses a magnetic shield that is formed in the shape of the entire CRT and display screen. A compensation coil is disposed around the outside of the shield at its open end on a plane parallel to the display screen. The compensation coil produces a bucking field, in response to a dc current, that opposes the external field. Four magnetic field sensors are positioned adjacent to the inner surface of the magnetic shield at the back of the CRT. The positions of the sensors makes them responsive only to the portion of the external magnetic field that enters the shield, and insensitive to the bucking field. The sensors must be very sensitive to detect the relatively weak fields that penetrate the shield, and hence are expensive. A circuit adjusts the dc current to the coil in response to the external field strength sensed by the four sensors. This system must be calibrated in a known external field.
U.S. Pat. No. 5,073,744, "Method and Apparatus For Dynamic Magnetic Field Neutralization" to Bubler, discloses four different and nonplanar coils positioned around the open end of a magnetic shield for producing bucking fields in opposition to the external field. Each coil has portions parallel to each of the axial, horizontal and vertical fields, and is in electrical communication with an expensive and complex fluxgate sensor. Four sensors are placed as close to the CRT display screen as possible in respective corners of the screen. The control circuitry adjusts the drive currents in the respective coils such that the resultant field at each sensor is reduced. The system requires calibration in a known field.
The currently available systems utilize complex and expensive sensors to detect the external fields, and require calibration in a known external field. A simple and inexpensive system that accurately senses and substantially opposes the external fields so that the display is not distorted is needed. The system should not require calibration, and should be able to use relatively insensitive, and thus inexpensive, magnetic field sensors. The shielding system should also be capable of automatically degaussing the display from the sensed field components.